Antenna equipment

Abstract: A rod antenna element (11) is connected at its lower end to one end of a coaxial impedance converter, the other end of which is connected to a feeder (14). A coil antenna element (16) is capacitively coupled to the rod antenna element to provide a double resonance characteristic. When held at its retracted position, the rod antenna element, inserted in an outer conductor of the coaxial impedance converter, may preferably form an inner conductor larger in diameter than that when the rod antenna element is at its extended-out position, and the rod antenna element is connected to a coil antenna element with a low impedance.

Description: BACKGROUND OF THE INVENTION The present invention relates to an antenna equipment for use with automobile, portable and cordless telephones and other mobile station radio units. The mobile radio communication network has been steadily extended to meet a growing demand for daily use and cannot be accommodated in a single frequency band conventionally assigned thereto; now, it is assigned one more frequency band. It is desired, therefore, that every mobile station equipment be switchable between these two frequencies -- this calls for an antenna equipment that resonates with two different frequencies. Figs. 1 and 2 show prior art examples of such an antenna equipment adapted for resonance with two frequencies. In the example of Fig. 1 a resonance circuit 7 is provided at a midpoint in an antenna element 11 and has a resonance frequency different from that of the antenna element 11, and besides, a matching circuit 8 is connected between a feeder 14 and the antenna element 11 to match their impedances. In the example of Fig. 2 the matching circuit 8 between the antenna element 11 and the feeder 14 is adapted to resonate with two frequencies. In the unit of Fig. 1 the matching circuit 8 is relatively simple in structure but the provision of the resonance circuit 7 at a midpoint in the antenna element 11 introduces complexity in the mechanical structure of the antenna equipment, and in general, the antenna element 11 readily becomes crimped at that portion. In the example of Fig. 2 the matching circuit 8 is complex in structure and the provision of such a complicated matching circuit 8 will increase the power loss or dissipation by the antenna circuit accordingly. Besides, in the prior art examples of Figs. 1 and 2 an antenna current develops in an antenna housing 9 (indicated by a symbol of ground potential); consequently, in a radio unit of the type that the housing is held by hand, the current distribution varies with how the housing is held and with the movement of the human body, causing a change in the radiation characteristic of the antenna. Furthermore, the antenna characteristic itself is also affected by the shape and material of the housing and parts mounted thereon (such as a dial pad and a liquid crystal display screen). In Japanese Patent Application Laid-Open No. 213303/87 there is disclosed an antenna equipment of a construction in which a coaxial line of a length λ/4 (λ being the wavelength used) and a characteristic impedance Z o is connected between the feeding point of a λ/2 rod antenna and a feeder of a characteristic impedance Zb, and the impedance Za of the antenna feeding point and the impedances Zb and Z o of the above-mentioned feeder and coaxial line are selected such that Z o = (ZaZb) 1/2 , thereby implementing the intended impedance matching. The antenna equipment of the above construction is capable of achieving high gains for wavelengths which are integral multiples of λ/2; besides, since the impedance of the antenna feeding point is very high (infinite, theoretically), the antenna current flowing to the housing is limited, and consequently, the dependence of the antenna characteristic on the housing structure is low and even if the housing is held by hand, the radiation characteristic of the antenna does not appreciably change. With the above-described antenna structure, however, a second operating wavelength is limited to integral multiples of λ/2 in contrast to the first wavelength λ, and hence it cannot freely be chosen. Moreover, it is difficult to achieve high gains for two wave-lengths which are relatively close to each other within λ/2 in the frequency band assigned to the mobile radio communication. Portable radio telephones utilize, in many cases, a telescopic antenna equipment of the type that the antenna element is extended out of the unit housing during communication but housed in the housing while not in use. In Japanese Patent Application Laid-Open No. 170201/89, for example, there is disclosed an antenna of a construction in which a first rod (0.6 λ) is received in a second rod (0.5 λ), which is received in a third rod, which is, in turn, disposed inside a metal pipe, thus forming a λ/4 long impedance matching coaxial line. Such a telescopic antenna equipment allows ease in carrying the radio telephone while not in use for communication, but the portable radio telephone needs to be held in the wait-receive mode in which to continue receiving electric waves from a base station at all times while not in use for communication, too. Hence, when the antenna element is retracted into and housed in the unit housing in the above-mentioned wait-receive mode, the impedance characteristic of the antenna will change, resulting in extreme reduction of its gain for received waves. In this instance, if the housing is made of metal, the sensitivity of the antenna will go down to substantially zero since it is covered with metal. Thus, it is impossible, in principle, to use such an antenna in its retracted state in the radio telephone that must be held in the wait-receive mode during the non-communication period. On the other hand, a diversity antenna requires two antenna elements, and hence is inevitably bulky. An antenna equipment according to the prior art portion of claim 1 is disclosed in the document GB-A-2 257 836. In this prior art, the diameter of the inner conductor is larger than that of the rod antenna element. The antenna element is provided at its bottom end with an impedance matching inductor, which is connected to the inner conductor. When the antenna element is retracted, its both ends are short-circuited via the inner conductor and two complementary collets. Thus, the antenna element including the impedance matching inductor is rendered inactive and the coaxial line formed by a dielectric tube, the inner conductor and the outer conductor) acts merely as a feed to a helical second antenna element. Since the rod antenna element is retracted into the cylindrical inner conductor, the inner conductor remains to act as the inner conductor of the coaxial line and, therefore, the impedance of the coaxial line does not change before and after the retraction of the rod antenna. SUMMARY OF THE INVENTION An object of the present invention is to provide an antenna equipment which resonates with a plurality of frequencies and is simple-structured and low-loss and whose radiation characteristic resists being affected by the human body or unit housing. Another object of the present invention is to provide an antenna equipment which, when retracted in the unit housing, has sensitivity to such an extent as to permit the wait-receive mode and whose radiation characteristic resists being affected by the human body or unit housing. Still another object of the present invention is to provide an antenna equipment which is very small when formed for diversity reception too. These objects are achieved with an antenna equipment as claimed in claims 1 and 9. Preferred embodiments of the invention are subject-matter of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram schematically showing an example a conventional antenna equipment in which a resonance circuit is connected to a rod-like antenna element to provide two resonance points; Fig. 2 is a diagram schematically showing an example of a conventional antenna equipment in which a resonance circuit is connected to a matching circuit connected to a rod-like antenna to provide two resonance points; Fig. 3A is a perspective view, partly in section, illustrating an embodiment of the present invention in which a rod antenna element and a coil antenna element are adapted to selectively operate, depending upon whether the antenna is held at its extended-out or retracted position, and the matching state or level of the coaxial type impedance converter is changed correspondingly; Fig. 3B is a perspective view, partly in section, showing the antenna equipment of Fig. 3A, with the antenna held at its retracted position; Fig. 3C is a longitudinal sectional view of Fig. 3A; Fig. 3D is a longitudinal sectional view of Fig. 3B; Fig. 4A is a graph 3 showing the impedance characteristic of the Fig. 3A embodiment when the antenna is held at its extended-out position; Fig. 4B is a graph showing the impedance characteristic of the Fig. 3A embodiment when the antenna is held at its retracted position; Fig. 5A is a perspective view, partly in section, illustrating a modified form of the Fig. 3A embodiment in which the inner conductor of the coaxial type impedance converter is made partly thick; Fig. 5B is a perspective view, partly in section, showing the Fig. 5A embodiment, with the antenna is held at its retracted position; Fig. 6A is a longitudinal sectional view illustrating another modified form of the Fig. 3A in which the coaxial type impedance converter is connected to an intermediate tap of the coil forming the coil antenna element when the antenna is held at the retracted position; Fig. 6B is a longitudinal sectional view showing the antenna equipment of Fig. 6A, with the antenna held at its retracted position; Fig. 7A is a graph showing the return-loss characteristic of the Fig. 6A embodiment with the antenna held at its projecting-out position; Fig. 7B is a graph showing the return-loss characteristic of the Fig. 6A embodiment with the antenna held at its retracted position; Fig. 8A is a longitudinal sectional view illustrating another embodiment of the present invention in which a quarterwave coil antenna element is connected to the tip of a quarterwave rod antenna element to form a half-way antenna; Fig. 8B is a longitudinal sectional view showing the antenna equipment of Fig. 8A with the antenna held at its retracted position; Fig. 9A is a longitudinal sectional view illustrating a modified form of the Fig. 8A embodiment in which the coil antenna element is electrically isolated from the rod antenna element and the former is connected to the coaxial type impedance converter when the antenna is held at its extended-out position; Fig. 9B is a longitudinal section view of the antenna equipment of Fig. 9A with the antenna held at its retracted position; Fig. 10A is a longitudinal sectional view illustrating a modified form of the Fig. 9A embodiment in which an inverted F antenna is connected to the coaxial-type impedance converter when the antenna is held at its extended-out position; Fig. 10B is a longitudinal sectional view showing the antenna equipment of Fig. 10A with the antenna held at its retracted position; Fig. 11A is a longitudinal sectional view illustrating another embodiment of the present invention in which the inner conductor of the coaxial type impedance converter is used as an antenna retracting guide; Fig. 11B is a longitudinal sectional view showing the antenna equipment of Fig. 11A with the antenna held at its retracted position; Fig. 12A is a perspective view illustrating an example of a diversity antenna embodying the present invention, with the antenna held at its extended-out position; Fig. 12B is a perspective view showing the Fig. 12A example, with the antenna held at its retracted position; Fig. 12C is a longitudinal sectional view of the Fig. 12A example; Fig. 12D is a longitudinal sectional view showing the diversity antenna with the antenna retracted; Fig. 13A is a graph showing the impedance characteristic of the rod antenna element in the Fig. 12A example when the antenna is held at its extended-out position; Fig. 13B is a graph showing the impedance characteristic of the slot antenna element when the antenna is held at its extended-out position; Fig. 14A is a graph showing the coupling characteristic of the rod and slot antenna elements when the antenna was held at its extended-out position; Fig. 14B is a graph showing the return-loss characteristic of the rod antenna element when the antenna is held at its retracted position; Fig. 15A is a diagram showing the relationships of the rod antenna element, the antenna housing, the measuring electric fields and the coordinates used for measuring the radiation patterns of the Fig. 12A example; Fig. 15B is a diagram showing the radiation pattern of the rod antenna element in the horizontal plane (X-Y); Fig. 15C is a diagram showing the radiation pattern of the rod antenna in the vertical plane (X-Z); Fig. 15D is a diagram showing the radiation pattern of the slot antenna element in the horizontal plane (X-Y); Fig. 15E is a diagram showing the radiation pattern of the slot antenna element in the vertical plane (X-Z); Fig. 16A is a perspective view illustrating a modified form of an antenna held at its extended-out position; and Fig. 16B is a perspective view showing the Fig. 16A example, with the antenna held at its retracted position. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3A is a perspective view illustrating an embodiment according to the invention, with the rod antenna element 11 pulled out from the housing 9, and Fig. 3B also a perspective view showing the state in which the rod antenna 11 is retracted in the housing 9. Figs. 3C and 3D are longitudinal sectional views corresponding to Figs. 3A and 3B. In this embodiment, the rod antenna element 11 is slidably received in the metal cylinder 12 along its center axis so that it may be pulled out therefrom as required. The fine wire 13 is extended substantially along the center axis of the metal cylinder 12, and in the lower end portion of the metal cylinder 12, the lower end of the fine wire 13 and the core conductor 14a of the feeder 14 are interconnected. Provided immediately above the metal cylinder 12 is a ring-shaped contact metal member 18 which receives the rod antenna element 11 and makes sliding contact therewith and to which the top end of the fine wire 13 is connected. The coil antenna element 16 is disposed outside the contact metal member 18 concentrically therewith, and when the rod antenna element 11 is retracted in the metal cylinder 12, the upper end of the coil antenna element 16 makes elastic contact with a metal disc 11C mounted on the top of the antenna element 11. To guide the rod antenna element 11 accurately along the axis of the metal cylinder 12, there is provided inside the metal cylinder 12 an insulating guide tube 19 coaxial therewith. The lower end of the insulating guide tube 19 is fixedly secured to an insulating support plate 19A (Figs. 3C and 3D) fitted into the lower end portion of the metal cylinder 12 and the fine wire 13 is extended in the axial direction of the insulating guide tube 19 and fixed to the outside thereof. The rod antenna element 11 is composed of a thin or linear first rod 11 1 having the metal disc 11C at its tip and a tubular second rod 11 2 which receives therein the first rod 11 1 . When guided into the insulating guide tube 19, the second rod 11 2 has retracted therein the first rod 11 1 . The length of the rod antenna element 11 is substantially equal to λ/2 at its extended-out position. When the rod antenna 11 is at its extended-out position as shown in Figs. 3A and 3C, it is necessary to match the 50-ohm impedance of the feeder 14 and an impedance of hundreds of ohms which is developed by feeding the half-wave rod antenna element 11 from its lower end. To perform this, a coaxial matching means (an impedance converter) is provided between the rod antenna element 11 and the feeder 14. The coaxial structure is made up of the metal cylinder 12 of about an quarter-wave length, forming the outer conductor of the coaxial structure, and the fine wire 13 forming the inner conductor. To set the characteristic impedance Zo of the coaxial structure to, for example, around 200 ohms, a value close to Zo = (ZaZb) 1/2 where the impedance Zb of the feeder 14 is 50 ohms and the impedance Za of the rod antenna element 11 is hundreds of ohms, the diameter ratio of the inner and outer conductors needs only to be 6. For example, when the diameter of the inner conductor is 1 mm, the diameter of the outer conductor is 6 mm. In this embodiment, since the fine wire 13 forming the inner conductor is wired along the outside surface of the insulating guide tube 19 which receives the rod antenna 11, the inner conductor is off the center axis of the outer conductor; nevertheless, a proper characteristic impedance can be obtained. When the rod antenna element 11 is held at its projecting-out position, the coil antenna 16 is completely isolated and its resonance wavelength deviates from the operating wavelength; consequently, the coil antenna element 16 has no effect on the operating characteristic of the rod antenna 11 at that time. When the rod antenna element 11 is retracted in the metal cylinder 12 as shown in Figs. 3B and 3D, the core 14a of the feeder 14 is connected to the rod antenna element 11 via a coiled elastic contact terminal C1 provided on bottom of the insulating guide tube 19. At the same time, the tip of the coil antenna element 16, which forms an elastic contact terminal C3, makes elastic contact with the metal disc 11 of the rod antenna element 11c, by which the coil antenna element 16 is connected to the rod antenna element 11. The coil antenna element 16 is designed to resonate with an impedance lower than does the rod antenna element 11. The rod antenna element 11, when retracted, functions as the inner conductor of the coaxial impedance converter 10. The rod antenna element 11 is larger in diameter than the fine wire 13 and the characteristic impedance of the coaxial structure goes low. For example, when the outer diameter of the rod antenna element 11 is 3 mm and the inner diameter of the metal cylinder 12 is 6 mm, the characteristic impedance of the coaxial structure is about 50 ohms. In this instance, the coaxial structure formed by the metal cylinder 12 and the rod antenna element 11 retracted therein operates as a mere 50-ohm transmission line, not as the impedance converter, and it is connected via the elastic contact terminal C3 to the coil antenna element 16 which operates with a low impedance. In this situation, the rod antenna element 11 does not ever exert any influence on the operating characteristic of the coil antenna element 16. When the rod antenna element 11 is held at its pulled-out position, the coaxial structure 10 serves as an impedance converter as described above, and consequently, received power can efficiently be provided to the feeder 14 from the high-impedance rod antenna element 11 which operates with a high gain as a half-wave antenna. On the other hand, when the rod antenna element 11 is retracted in the metal cylinder 12, the coaxial structure 10 performs the function of a 50-ohm transmission line as an extension of the feeder 14, and hence received power can efficiently be taken out from the low-impedance coil antenna element 16 which operates as a quarter-wave antenna. While in the above the rod antenna element 11 has a length substantially equal to the half-wave length and the metal cylinder 12 a length equal to the quarter-wave length, the length of the rod antenna element 11 may also be chosen at will, in which case the length and characteristic impedance of the coaxial structure 10 need only to be selected appropriately. Also in this embodiment, when the rod antenna element 11 is held at the pulled-out position, the metal cylinder 12 acts as a stub and prevents a current flow to the casing 9, and hence the rod antenna element is hardly affected by the casing on which the antenna equipment is amounted; furthermore, since the coaxial impedance converter formed by distributed constant is used as the matching circuit, the bandwidth is wide and high gains can be obtained. In Figs. 4A and 4B there are shown impedance characteristics of the coaxial impedance converter 10 measured when the rod antenna element 11 was held at its pulled-out and retracted positions in the Figs. 3A, 3B embodiment. The metal cylinder 12 was 5 cm in length and 1 cm in diameter; the rod antenna element 11 was 10 cm long; the coil antenna element 16 was 1 cm in diameter and its number of turns was 2.5; and the antenna equipment was mounted on the metal casing of a volume about 200 cc. As seen from Figs. 4A and 4B, the antenna equipment resonated at 1.44 GHz when the rod antenna element 11 was at the pulled-out position and at 1.46 GHz when the antenna rod 11 was at the retracted position; that is, the antenna equipment resonated at about the same frequency. This reveals that when the rod antenna element 11 is at the extended-out position, it is 10 cm long and functions as a half-wave antenna and that when the rod antenna 11 is at the retracted position, the coil antenna element 16 serves as a quarter-wave antenna because its length is about 2.5 cm. From this, it is seen that the characteristic impedance of the coaxial impedance converter changes with the position of the rod antenna element 11 and that received power at each resonance point can efficiently be taken out. The receiving bandwidth in the case of the rod antenna element 11 being at the pulled-out position is 150 MHz with VSWR < 2 and the specific bandwidth is as wide as more than 10%, and the gain is also about the same as that of a half-wave dipole antenna. Fig. 5A illustrates, in perspective, a second embodiment of the invention, with the rod antenna element 11 held at the extended-out position, and Fig. 5B also illustrates, in perspective, the state in which the rod antenna element 11 is retracted. This embodiment is identical in construction with the Fig. 3 embodiment except that a conductive pipe 13A is fitted in the lower end portion of the nonconductive guide tube 19 coaxially therewith. The conductor pipe 13A has about the same diameter as that of the insulating guide tube 19 which receives therein the rod antenna element 11. The conductor pipe 13A has its lower end connected to the inner conductor 14a of the feeder 14 and its upper end connected to the fine wire 13. When the rod antenna element 11 is at the retracted position, the lower end portion of the its second rod 11 2 is inserted in the conductor pipe 13A and constitutes the inner conductor of the low impedance coaxial line in combination with the conductor pipe 13A. At this time, the contact terminal C3 of the coil antenna element 16 is connected via the metal disc 11C to the inner conductor of the coaxial line 10 as in the case of the Fig. 3 embodiment. When the rod antenna element 11 is held at the extended-out position, the coaxial structure 10 using the metal cylinder 12 as the outer conductor is made up of a part using the fine wire 13 as the inner conductor and a part using as the inner conductor the conductor pipe 13A connected in series to the fine wire 13. Since the two parts have different characteristic impedances, the impedance converter can be designed with a higher degree of freedom. That is, the provision of such a two-stage impedance converter allows ease in achieving the double resonance characteristic and permits widening the band of the antenna characteristic. When the characteristic of the part using the conductor pipe 13A as the inner conductor is set to 50 ohms, only the part in which the fine wire 13 serves as the inner conductor operates as an impedance converter; thus, it is possible to change the length of the impedance converter part alone while holding the length of the metal cylinder 12 unchanged at the quarter-wave length. Also in this instance, when the rod antenna element 11 is held at the retracted position, the conductor pipe 13A and the second rod 11 2 received therein form a unitary structure with each other. This state is identical with that shown in Figs. 3B and 3D and the principle of operation is also the same. Thus, the Fig. 5 embodiment achieves high gains regardless of whether the rod antenna element 11 is at the extended or retracted position and implements a wide band characteristic. Fig. 6A is a longitudinal sectional view, partly in section, of a third embodiment according to the invention, with the rod antenna element 11 held at the extended position, and Fig. 6B a longitudinal sectional view showing the state in which the rod antenna element 11 is at the retracted position. This embodiment is identical in construction with the Fig. 3 embodiment except that the contact terminal C3 is connected to an intermediate tap 16T of the coil forming the coil antenna element 16 and that the capacitor 15 is connected between the top end of the coil antenna 16 and the ring-shaped contact metal member, as required. Accordingly, when the rod antenna element 11 is retracted in the metal cylinder 12, the tap 16T of the coil antenna element 16 makes contact with the metal disc 11C mounted on the tip of the rod antenna element 11. When the rod antenna element 11 of the two-stage structure formed by the first and second rods 11 1 and 11 2 is at the extended position, its length is about λ /2 and the length of the metal cylinder 12 is about λ/4. With such a structure of this embodiment, when the rod antenna element 11 is at the extended position, a resonance circuit made up of the coil antenna element 16 and the capacitor 15 is provided in parallel to the rod antenna element 11, by which the 2-resonance characteristic can be obtained. When the rod antenna element 11 is retracted in the casing 9, the metal disc 11C and contact terminal C3 contact each other and the tap 16T of the coil antenna element 16 is connected via the antenna element 11 to the feeder 14, and consequently, the coil antenna element 16 serves as a quarterwave radiation element of one resonance characteristic. In this case, the coil part from the top end portion of the coil antenna element 16 to the tap 16T becomes shorted and draws substantially no current. Fig. 7A is a graph showing the return-loss characteristic measured when the rod antenna element 11 shown in Fig. 6A was at the extended position, f1 and f2 being resonance frequencies. Fig. 7B is a graph showing the return-loss characteristic measured when the rod antenna 11 was at the retracted position, f3 being a resonance frequency. The metal cylinder 12 was 8 cm long and 1 cm in diameter; the maximum length of the rod antenna element 11 was 15 cm; the coil antenna element 16 was 1 cm in diameter and its number of turns was 3; the capacitance of the capacitor 15 was about 1 pF; and the antenna equipment was mounted on a casing of a volume about 200 cc. As shown in Fig. 7A, a 2-resonance characteristic that the antenna resonates at f1 = 835 MHz and f2 = 1005 MHz was obtained. As shown in Fig. 7B, when the rod antenna 11 was at the retracted position, a characteristic that the antenna resonates at f3 = 990 MHz was obtained by connecting the tap 16T to the portion of the coil antenna element 16 where the number of turns was about 2.5. Thus, by selecting the number of turns of the coil antenna element 16, the capacitance value of the capacitor 15 and the position of connection of the tap 16T, it is possible to obtain the 2-resonance characteristic when the rod antenna element 11 is at the extended position and a single resonance characteristic when the rod antenna 11 is at the retracted position. Fig. 8A is a sectional view illustrating a fourth embodiment of the invention, with the rod antenna element 11 held at the extended position, and Fig. 8B a sectional view showing the state in which the rod antenna 11 is retracted. In this embodiment, as in the embodiments of Figs. 3, 5 and 6, when the rod antenna 11 is at the extended position, the coaxial impedance converter 10 formed by the metal cylinder 12 of a length substantially equal to the half-wave length and the fine wire 13 is connected between the rod antenna element 11 and the feeder 14, and when the rod antenna 11 is at the retracted position, the coaxial line 10 by the rod antenna element 11 and the metal cylinder 12 serves as a transmission line of about the same low impedance as that of the feeder 14. This embodiment differs from the embodiments of Figs. 3, 5 and 6 in that the length of the rod antenna element 11 is substantially equal to the quarter-wavelength and that the coil antenna element 16 is connected to the tip of the rod antenna element 11 instead of being provided immediately above the metal cylinder 12. When the rod antenna element 11 is at the extended position, the coil antenna element 16 operates as a half-wave antenna in cooperation with the rod antenna element 11, whereas when the rod antenna 11 is at the retracted position in the metal cylinder 12, the coil antenna element 16 is positioned just above the metal cylinder 12 and operates as a quarter-wave antenna. Figs. 9A and 9B illustrate longitudinal sectional views illustrating a fifth embodiment of the antenna equipment according to the present invention. This embodiment is common to the Fig. 8 embodiment in the provision of the same coaxial impedance converter but differs therefrom in that the rod antenna 11 is composed of first and second rods 11 1 and 11 2 and has a length equal to the half-wavelength when it is extended and that the quarter-wave coil antenna element 16 is mounted on the tip of the first rod 11 1 but electrically isolated therefrom. When the rod antenna element 11 is retracted in the metal cylinder 12, the contact terminal C3 at the lower end of the coil antenna element 16 contacts the contact metal member 18, and hence is connected to the low-impedance coaxial line using the second rod 11 2 as the inner conductor. With the above antenna structure, when the rod antenna element 11 is at the extended position, only the rod antenna element 11 operates as a half-wave antenna, whereas when the rod antenna element 11 is at the retracted position, only the coil antenna element 16 operates as a quarter-wave antenna. Figs. 10A and 10B are longitudinal sectional views of a sixth embodiment which is a modified form of the Fig. 9 embodiment. In this embodiment, the coil antenna element 16 is substituted with an inverted F antenna element 32 mounted on the casing 9 and connected via a feeder 31 to the elastic contact terminal C3 provided near the contact metal member 18. When the rod antenna element 11 is at the retracted position, the metal disc 11C mounted on the tip of its first rod 11 1 contacts the contact terminal C3, connecting the inverted F antenna element 32 to the retracted rod antenna element 11 which forms the inner conductor of the low impedance coaxial line. The above-described embodiments of Figs. 3, 5, 6, 8, 9 and 10 employ the insulating guide tube 19 for guiding the rod antenna element 11 to the retracted position, and hence have a defect that the fine wire 13 is inevitably disposed off the center axis of the metal cylinder 12. In these embodiments, however, as shown in Figs. 11A and 11B, the insulating guide tube 19 need not always be provided and the metal fine wire 13 fixed at the lower end to the insulating support plate 19A may be disposed, also as a guide, along the center axis of the metal cylinder 12. The fine wire 13 is an elastic wire, and when the rod antenna element 11 formed by a tubular member of metal is at the extended position, the top end portion of the wire 13 still remains in the tubular member of the antenna element 11 and makes sliding contact therewith. In this embodiment, the cylindrical insulating holder 17 has a large-diameter portion whose inner diameter is nearly equal to the outer diameter of the metal cylinder 12 and a small-diameter portion which projects upwardly from the larger-diameter portion and whose outer diameter is smaller than that of the metal cylinder 12, and the large-diameter portion is fitted in the top end portion of the metal cylinder 12 coaxially therewith. The coil antenna element 16 is disposed around the small-diameter portion of the holder 17 and the upper end portion of the antenna element 16 projects upwardly of the top of the holder 17. When the rod antenna element 11 is retracted in the metal cylinder 12, the metal disc 11C and the contact terminal C3 at the tip of the coil antenna element 16 make elastic contact with each other. When the structure of this embodiment in which the fine wire 13 is inserted in the tubular member of the rod antenna element 11 to serve as a guides is applied to the above-described embodiments which have the rod antenna element 11 composed of the first and second rods 11 1 and 11 2 , it is needless to say that the first rod 11 1 is formed by a tubular member of metal to permit the insertion thereinto of the fine wire 13 when the rod antenna element 11 is retracted into the metal cylinder 12. This structure is applicable as well to the embodiments described below with reference to Figs. 12 and 16. Figs. 12A through 12D illustrate a seventh embodiment of the antenna equipment according to the present invention, in which the slot antenna 20 is provided in the Fig. 5 embodiment to form a small diversity antenna for use with portable radios which achieves high gains even when the rod antenna element 11 is at the retracted position. The casing 9 is made of a dielectric material such as a synthetic resin. On the outside of the upper small-diameter portion of the insulating holder 17 mounted on the top of the metal cylinder 12, there is disposed the coil antenna element 16 virtually coaxially with the rod antenna element 11. When the rod antenna element 11 is at the extended position, the coil antenna element 16 is isolated from the rod antenna element 11 and the impedance converter 10. A tubular sliding contact member 18 made of metal is fitted in the tubular insulating holder 17, with the axis of the former substantially aligned with the axis of the outer conductor 12, and the rod antenna element 11 is slidably received in the tubular sliding contact member 18. The rod antenna element 11 has at its lower end a flange 11B to prevent it from coming off the tubular sliding contact member 18. The small-diameter portion 13a of the inner conductor 13 is connected to the tubular sliding contact member 18 and is electrically connected therethrough to the rod antenna element 11. The length of the coil antenna element 16 over the entire coil is selected nearly equal to the quarter-wave length. The rod antenna element 11 has a length substantially equal to the half-wave length when it is extended. The coil antenna element 16 and the metal disc 11C need only to be electrically connected, and hence need not always be mechanically contacted. Therefore, power may be supplied to the coil antenna element 16 through utilization of the proximity capacitance by the coil antenna element 16 and the metal disc 11C slightly spaced apart. In this state of contact, the inner end of the rod antenna element 11 stays in the large-diameter portion 13b of the inner conductor 13 and the rod antenna element 11 is electrically connected via the large-diameter portion 13b to the feeder 14, with the result that the coil antenna element 16 is excited via the rod antenna element 11. In this embodiment, the flange 11B attached to the lower end of the rod antenna element 11 butts against the blocking end plate of the large-diameter portion 13b to limit further downward movement of the rod antenna element 11. In this example, the rod antenna element 11 is telescopic and its second rod 112 near the impedance converter 10 is tubular and the first rod 11 1 is smaller in diameter than the second rod 11 2 so that the former can be slid into and out of the latter. In the illustrated embodiment, the coil antenna element 16 is disposed in a truncated conical portion 9b protruded from the top panel 9a of the casing 9. The coaxial impedance converter 10 is fixed to the casing 9 in the inside thereof to secure thereto the antenna equipment. The feeders 14 and 24 are connected to receiving portions 30 and 35 in the casing 9 and the received outputs are diversity-combined in a combining part, though not shown. Although in the above-described embodiments the length of the rod antenna element 11 and the length of the outer conductor 12 have been described to be about λ/2 and λ/4, respectively, the length of the rod antenna element 11 may be arbitrary, in which case the length and characteristic impedance of the coaxial impedance converter 10 need only to be properly chosen in accordance with the length of the rod antenna element 11. For example, it is possible to select the length of the rod antenna element 11 to be 0.7 λ and direct it upward about 30 degrees at maximum in the vertical plane containing the rod antenna element 11, or to select the length of the rod antenna element 11 to be 0.3 λ and direct it downward about 30 degrees at maximum. Incidentally, the direction of the maximum directivity of the rod antenna element 11 having a length of 0.5 λ in the vertical plane is the horizontal direction (the lateral direction). In Figs. 13 through 15 there are shown the results of experiments conducted with the antenna equipment of the Fig. 12 embodiment. The values shown in Figs. 13 through 15 are impedance characteristics measured in the case where the outer conductor 12 was 5 cm long and 1 cm in diameter, the rod antenna element 11 was 10 cm long, the coil antenna element 16 was 1 cm in diameter and had a number of turns of 2.5, the slit 12G was 5 cm long and 3 mm wide, the capacitor 21 had a capacitance of about 1 pF and the coaxial impedance converter 10 was disposed in a dielectric casing 9 of a volume about 200 cc. Fig. 13A shows the return-loss characteristic of the rod antenna element 11 when it was extended, Fig. 13B the return-loss characteristic of the slot antenna 20 when the rod antenna element 11 was at the extended position, Fig. 14A the coupling characteristic of the rod antenna element 11 and the slot antenna 20 when the former was at the extended position, and Fig. 14B the characteristic of the rod antenna element 11 when it was at the retracted position. Fig. 13A and 13B show that when the rod antenna element 11 is at the extended position, it resonates with a frequency of about 1.44 GHz and the slot antenna 20 resonates with a frequency of about 1.49 GHz; their coupling is around 9 dB at maximum and when the rod antenna element 11 is retracted, it resonates with a frequency of about 1.46 GHz. That is, it was experimentally demonstrated that when the rod antenna element 11 is at the extended position, the rod antenna element 11 and the slot antenna 20 can be made to resonate independently of each other, though they share the same space, that their coupling is about 9 dB and that the rod antenna element 11 can be made to resonate with an arbitrary frequency even when it is at the retracted position. Figs. 15B through 15E show the radiation patterns measured when the rod antenna element was held at the extended position. In Fig. 15A there are shown the relationships among the casing 9, the rod antenna element 11, the coordinate axes X, Y and Z, the electric field E  emanating from the Z axis along a spherical surface with its center at the origin O and the electric field E  along a circle in the X-Y plane with its center at the origin 0. Fig. 15B shows the radiation pattern of the rod antenna element 11 in the horizontal plane (X-Y plane), Fig. 15C the radiation pattern of the rod antenna element 11 in the vertical plane (Y-Z plane), Fig. 15D the radiation pattern of the slot antenna 20 in the horizontal plane (X-Y plane) and Fig. 15E the radiation pattern of the slot antenna 20 in the vertical plane (X-Z plane). As depicted in Figs. 15B and 15C, the radiation pattern of the rod antenna element 11 in the horizontal (X-Y) plane is virtually round and the radiation pattern in the vertical plane is close to an 8-letter shaped pattern, and the radiation level is about the same as that of a half-wave dipole antenna. This reveals that the rod antenna element 11 acts as a half-wave antenna and suffers practically no loss. The slot antenna 20 has a relatively unidirectional pattern in the horizontal plane and the radiation level is lower about 3 dB than the dipole antenna. Furthermore, the correlation function of the both antennas measured outdoors was below 0.6 although they shared the same space. From the radiation patterns and the measured value of the correlation function, it is seen that the diversity effect is also satisfactory. Thus, this antenna structure permits the implementation of an antenna equipment which has high gains and a wide-band characteristic, lessens the influence of the antenna casing and achieves high gains when the rod antenna element is at the retracted position and which can be made very small as a diversity antenna. Figs. 16A and 16B illustrate an eighth embodiment of the antenna equipment according to the present invention. In this embodiment, when it is at the extended position, only the rod antenna element 11 operates as an antenna, whereas when the antenna element 11 is at the retracted position, only the slot antenna 20 operates as an antenna. As is the case with the Fig. 12 embodiment, the rod antenna element 11 is slidably received in the coaxial impedance converter 10. In this embodiment, the insulating guide tube 19 is extended almost all over the length of the outer conductor 12. Furthermore, the tubular sliding contact member 18 is also provided to slidably receive the rod antenna element 11. In this embodiment, the other end of the feeder 24 for the slot antenna 20 is connected in parallel to the feeder 14 at the junction point of the impedance converter 10 and the feeder 14. The length of the impedance converter 10 is selected substantially equal to the quarter-wave length. Besides, a short-circuit means 11C is provided to connect the projecting end of the rod antenna element 11 to the outer conductor 12 when the rod antenna element 11 is at the retracted position. In the illustrated example, the top end portion of the rod antenna element 11 is bent substantially at right angles to form the short-circuit means 11C. To ensure good contact of the short-circuit means 11C with the outer conductor 12, a small contact piece 12C is extended from the marginal edge of the outer conductor 12 near the rod antenna element 11 toward the inner conductor 12 so that the short-circuit means 11C goes down into contact with the small contact pieces 12C when the rod antenna element 11 is retracted. To prevent the rod antenna element 11 from turning about its axis, its flange 11B (see Figs. 12C and 12D), for example, is partly cut off and a ridge is formed on the interior surface of the guide tube 19 in its axial direction so that it slides into engagement with the notch of the flange 11B. The capacitance of the capacitor 21 is chosen so that when the rod antenna element 11 is at the retracted position, the slot antenna 20 resonates with a desired frequency and so that the impedance at the side of the feeder 24 viewed from the connection point of the feeders 14 and 24 becomes equal to the 50-ohm characteristic impedance of the coaxial cable. When the rod antenna element 11 is at the extended position, the resonance frequency of the slot antenna 20 is low and the frequency band is narrow; therefore, the impedance at the side of the feeder 24 viewed from the connection point of the feeders 14 and 24 is made appreciably high. Consequently, when the rod antenna element 11 is extended, the impedance of the slot antenna 20 viewed from the connection point of the feeders 14 and 24 is markedly high and only the impedance of the rod antenna element 11, converted by the coaxial impedance converter 10 to 50 ohms, is observed and the rod antenna element 11 radiates. On the other hand, when the rod antenna element 11 is retracted, the coaxial impedance converter 10 viewed from the connection point of the feeders 14 and 24 becomes a λ /4 short-circuit line and provides an infinite impedance, since the tip of the converter 10 is short-circuited by the short-circuit means 11C. However, since the slot antenna 20 is matched to 50 ohms, power is fed to the slot antenna 20 via the feeder 14 and the slot antenna 20 radiates. This antenna structure can be applied to a diversity antenna by forming two slits and using one of them as a slot antenna exclusively for the diversity antenna. Thus, this antenna structure permits the implementation of an antenna equipment which has high gains and a wide-band characteristic, lessens the influence of the antenna casing and achieves high gains when the rod antenna element is at the retracted position and which can be made very small as a diversity antenna.

Claims: An antenna equipment comprising: a metal cylinder (12); an inner conductor (13) extended in said metal cylinder along its center axis to form a coaxial line in combination with said metal cylinder; a rod antenna element (11) as a first antenna element which is moveable along the center axis of the metal cylinder between a projecting state where it projects out from said metal cylinder and a retracted state where it is retracted into the metal cylinder; and a second antenna element (16) which is connected to said rod antenna element when said rod antenna element is in its retracted state; characterized by sliding-contact means (18) for causing one end of said inner conductor to make sliding contact with said rod antenna element, said rod antenna element having a diameter larger than that of said inner conductor; and a feeder (14) having a core conductor (14a) connected to said inner conductor and an outer conductor (14b) connected to said metal cylinder at one end thereof opposite from said rod antenna element; wherein when said rod antenna element is in its retracted state, the inner end of said rod antenna element makes contact with said core conductor (14a) of said feeder to form, together with said metal cylinder, a coaxial line that has substantially the same impedance as said feeder and connects said second antenna element with said feeder, while when said rod antenna element is in its projecting state, said second antenna element is disconnected from said rod antenna element and said metal cylinder and said inner conductor constitute a coaxial impedance converter which matches the impedances of said rod antenna element and said feeder and interconnects them. The antenna equipment of claim 1, wherein said second antenna element (16) is a coil antenna element disposed at the top of said metal cylinder (12) in a manner to surround a part of said rod antenna element (11), said rod antenna element has near its top end a contact terminal (11C) extending therefrom at right angles to its axial direction, and said contact terminal makes contact with said coil antenna element when said rod antenna element is retracted in said metal cylinder. The antenna equipment of claim 2, wherein said coil antenna element (16) has an intermediate tap (16T) which makes contact with said contact terminal (11C) when said rod antenna element (11) is retracted in said metal cylinder. The antenna equipment of claim 3, wherein one end of said coil antenna element (16) is connected via a capacitor (15) to said sliding-contact means (18). The antenna equipment of claim 1, wherein said inner conductor (13) has a tubular large-diameter portion for the portion connected to said feeder (14) and said rod antenna element retracted in said metal cylinder is inserted into said tubular large-diameter portion of said inner conductor. The antenna equipment of claim 1, wherein said second antenna element (16) is a coil antenna element disposed at the top end of said rod antenna element but electrically insulated therefrom. The antenna equipment of claim 1, wherein said second antenna element is an inverted F antenna element (32) disposed near the top end of said metal cylinder (12). The antenna equipment of claim 1, wherein said rod antenna element (11) comprises first and second rods (11 1 , 11 2 ) one of which is retractable into the other, said rod antenna element has a length of about one-half the wavelength used when extended and the length of said metal cylinder (12) is about a quarter of said wavelength used. An antenna equipment comprising: a metal cylinder (12); an inner conductor (13) extended in said metal cylinder along its center axis and forming a coaxial line in combination with said metal cylinder; a rod antenna element (11) as a first antenna element which is moveable along the center axis of the metal cylinder between a projecting state where it projects out from said metal cylinder and a retracted state where it is retracted into the metal cylinder; and a coil antenna element (16) as a second antenna element which is connected at one end to the tip of said rod antenna element; characterized by sliding-contact means (18) for causing one end of said inner conductor to make sliding contact with said rod antenna element, said rod antenna element having a diameter larger than that of said inner conductor; and a feeder (14) having a core conductor (14a) connected to said inner conductor and an outer conductor (14b) connected to said metal cylinder at one end thereof opposite from said rod antenna element; wherein when said rod antenna element is in its retracted state, said coil antenna element projects out from the top end of said metal cylinder and the inner end of said rod antenna element makes contact with said core conductor of said feeder to form, together with said metal cylinder, a coaxial line that has substantially the same impedance as said feeder and connects said coil antenna element with said feeder; while when said rod antenna element is in its projecting state, said rod and coil antenna elements, together, form a single antenna and said metal cylinder and said inner conductor constitute a coaxial impedance converter which matches the impedances of said rod antenna element and said feeder and interconnects them. The antenna equipment of claim 9, wherein said rod antenna element (11) has a length substantially equal to a quarter of the wavelength used and said coil antenna element (16) has a resonance point at said wavelength used. The antenna equipment of claim 1 or 9, wherein an insulating guide tube (19) for guiding and retracting thereinto said rod antenna element (11) is provided in said metal cylinder (12), the center axis of the guide tube and that of the metal cylinder being held in alignment with each other, said inner conductor (13) is extended over the outer peripheral surface of said guide tube in its axial direction and said sliding-contact means (18) is a metal piece which is connected to one end of said inner conductor and makes sliding contact with said rod antenna element. The antenna equipment of claim 11, wherein said metal piece forming said sliding-contact means (18) is an annular member and said rod antenna element (11) is inserted thereinto for sliding contact therewith. The antenna equipment of claim 1 or 9, wherein said rod antenna element (11) is a tubular element, said inner conductor (13) is an elastic wire disposed along the center axis of said metal cylinder (12) and having its top end portion inserted into said tubular element, for guiding said rod antenna element when it is retracted into said metal cylinder, the tip end portion of said elastic wire forming said sliding-contact means which makes sliding contact with said rod antenna element in said tubular element. The antenna equipment of claim 1 or 9, wherein said metal cylinder (12) has a slit (12G) formed therein in its axial direction to form a slot antenna, and a core conductor and an outer conductor of another feeder (24) are connected to opposed marginal edges of said metal cylinder across said slit. The antenna equipment of claim 14, wherein a capacitor (21) for frequency adjusting use is connected between said opposed marginal edges of said metal cylinder (12) across said slit (12G). The antenna equipment of claim 1, wherein said metal cylinder (12) has a slit (12G) formed therein along its axial direction to form a slot antenna; and wherein a second feeder (24) is connected at one end to said slot antenna. The antenna equipment of claim 16, wherein said second antenna element (16) is a coil antenna element disposed around a part of said rod antenna element (11) coaxially therewith near the top end portion of said metal cylinder (12), said coil antenna element being capacitively coupled to said rod antenna element when said rod antenna element is retracted in said metal cylinder. The antenna equipment of claim 16, wherein said second antenna element (16) comprises a coil antenna element (16) disposed around a part of said rod antenna element (11) coaxially therewith near the top end portion of said metal cylinder (12), said coil antenna element being electrically insulated from said rod antenna and said metal cylinder; said sliding-contact means (18) is connected to the tip of said inner conductor (13) and makes sliding contact with said rod antenna element; and a contact terminal (11C) extends from the tip of said rod antenna element at right angles to its axis and making contact with one end of said coil antenna element when said rod antenna element is retracted in said metal cylinder. The antenna equipment of claim 18, wherein an insulating guide tube (19) is disposed in said metal cylinder (12) substantially along its center axis for guiding said rod antenna element (11) inserted thereinto, and wherein said inner conductor (13) is extended over the outer peripheral surface of said guide tube in its axial direction. The antenna equipment of claim 19, wherein said sliding contact means (18) is an annular metal member which holds said rod antenna element (11) inserted thereinto. The antenna equipment of claim 16, wherein said rod antenna element (11) has at its tip a short-circuit portion (11C) which contacts said metal cylinder (12) when said rod antenna element is retracted in said metal cylinder; the other end of said second feeder (24) is connected in parallel to said first feeder (14); and the length of said second feeder is selected such that the impedance at the side of said second feeder, viewed from the connection point of said first and second feeders, is appreciably high when said rod antenna element is extended out from said metal cylinder and low when said rod antenna element is retracted in said metal cylinder. The antenna equipment of claim 16, 18 or 21, wherein that portion of said inner conductor (13) near said first feeder (14) is larger in diameter than that portion of said inner conductor near said rod antenna element (11). The antenna equipment of claim 16, 18 or 21, wherein the length of said rod antenna element (11) is about one-half the operating wavelength and the length of said metal cylinder (12) in its axial direction is about a quarter of said operating wavelength used. The antenna equipment of claim 16, 18 or 21, wherein a capacitor (22) is connected in parallel to the connection point of said first feeder (14) and said coaxial line. The antenna equipment of claim 18, wherein said rod antenna element (11) is a tubular element and said inner conductor (13) is an elastic wire disposed along the center axis of said metal cylinder (12) and having its tip inserted into the tubular element of said rod antenna element, said elastic wire sliding in said tubular element of said rod antenna element to guide it when said rod antenna element is retracted in said metal cylinder and said tip of said elastic wire forming said sliding-contact means which makes sliding contact with said rod antenna element in its tubular element.

Inventors: KOICHI TSUNEKAWA, SEIJI HAGIWARA, KOICHI, TSUNEKAWA, SEIJI, HAGIWARA
Applicants: NIPPON TELEGRAPH & TELEPHONE, NTT MOBILE COMMUNICATIONS NETWORK INC.
Application year: 1994
Main classification: H01Q5
IPC codes: H01Q5, H01Q1